The invention relates generally to semiconductor manufacturing, and more specifically to a method to mathematically characterize a multizone carrier used for retaining and pressing a semiconductor wafer against a polishing pad in a chemical-mechanical polishing tool.
A flat disk or xe2x80x9cwaferxe2x80x9d of single crystal silicon is the basic substrate material in the semiconductor industry for the manufacture of integrated circuits. Semiconductor wafers are typically created by growing an elongated cylinder or boule of single crystal silicon and then slicing individual wafers from the cylinder. The slicing causes both faces of the wafer to be extremely rough. The front face of the wafer on which integrated circuitry is to be constructed must be extremely flat in order to facilitate reliable semiconductor junctions with subsequent layers of material applied to the wafer. Also, the material layers (deposited thin film layers usually made of metals for conductors or oxides for insulators) applied to the wafer while building interconnects for the integrated circuitry must also be made a uniform thickness.
Planarization is the process of removing projections and other imperfections to create a flat planar surface, both locally and globally, and/or the removal of material to create a uniform thickness for a deposited thin film layer on a wafer. Semiconductor wafers are planarized or polished to achieve a smooth, flat finish before performing process steps that create integrated circuitry or interconnects on the wafer. A considerable amount of effort in the manufacturing of modern complex, high density multilevel interconnects is devoted to the planarization of the individual layers of the interconnect structure. Nonplanar surfaces create poor optical resolution of subsequent photolithography processing steps. Poor optical resolution prohibits the printing of high density lines. Planar interconnect surface layers are required in the fabrication of modern high density integrated circuits. To this end, CMP tools have been developed to provide controlled planarization of both structured and unstructured wafers.
A carrier in a CMP tools is used to retain a wafer and press against the back surface of the wafer so that the front surface of the wafer is pressed against a polishing pad. Slurry may be used to enhance the removal rate or planarity of the process. The amount of pressure at each point on the back surface of the wafer directly affects the amount of pressure between each point on the front surface of the wafer and the polishing pad. This relationship is important because the polishing removal rate at each point on the front surface of the wafer is proportional to the pressure on that point.
In general, it is desirable to remove material from the front surface of the wafer in a substantially uniform manner by applying a uniform pressure on the back surface of the wafer. However, thickness variations in incoming wafers, nonuniform slurry distribution, different motions for different points on the front surface of the wafer and other problems cause nonuniform planarization results. The nonuniform planarization results are typically manifested as concentric bands on the front surface of the wafer were greater or lesser amounts of material were removed. It may therefore be desirable to have different pressures on different concentric bands to compensate for the nonuniform removal rate.
Carriers able to provide different pressures on different concentric bands on the back surface of the wafer are referred to as multizone carriers. Multizone carriers can affect the polishing removal rate by applying different polishing pressures on different zones thereby creating a pressure distribution profile. Multizone carriers are typically able to apply different pressures on different zones by having two or more plenums that may be individually pressurized. The individually pressurized plenums press against the back surface of the wafer in order to control the pressures on the front surface of the wafer. The pressures between the front surface of the wafer and the polishing pad control the polishing removal rate.
However, Applicant has discovered that the pressures placed on the back surface of the wafer by a multizone carrier do not directly correspond to the pressures between the front surface of the wafer and the polishing pad. This is particularly true for transition areas between zones. For example, while a sharp pressure differential may exist between plenums, i.e. zones, pressing on the back surface of the wafer, a relatively smooth pressure transitional area will exist on the front surface of the wafer. The pressures on the front, not the back, surface of the wafer control the material removal rate. It is therefore highly desirable to be able to predict the pressure on the front surface of the wafer knowing the applied pressures on the back surface of the wafer. However, there is no conventional method for predicting the pressure profile on the front surface of the wafer, particularly within the transitional area, knowing the pressures applied to the back surface of the wafer. In addition, there is no conventional method for determining the combination of pressures needed in the zones to optimize the required polishing removal profile.
What is needed is a method to mathematically characterize a multizone carrier so that the optimum combination of pressures may be determined and applied to different zones on the back surface thereby creating a desired removal rate profile on the front surface of the wafer.
The invention is a method for mathematically characterizing a multizone CMP carrier. This allows a material removal profile to be calculated for a particular multizone carrier given a combination of pressures for the zones within the carrier.
In a preferred embodiment, alternating zones are pressurized to a first pressure (Pmax) and the remaining zones are pressurized to a second lower pressure (Pmin). For example, pressures of Pmax, Pmin and Pmax may be used for zones 1, 2 and 3 respectively in a three-zone carrier. A first wafer may then be polished using this combination of pressures and a first material removal profile may be found for the first wafer using this combination of pressures.
The pressures in the zones may then be reversed, i.e. zones with Pmax are given Pmin and zones with Pmin are given Pmax. For example, pressures of Pmin, Pmax and Pmin may be used for zones 1, 2 and 3 respectively in a three-zone carrier. A second wafer may then be polished using this new combination of pressures and a second material removal profile may be found for the second wafer using this combination of pressures.
The data from the first and second material removal profiles is preferably normalized to assist in the mathematical analysis. Points of intersection between the first and second material removal profiles may be found which identify a radius of a zone and a middle point in a transitional area between zones. Each zone, except for the outermost zone, will have two points of intersection identifying the length and position of the diameter for that zone. The two points for each zone, assuming the multizone carrier has symmetrical plenums, may easily be identified because the two points will be roughly symmetrical with each other about the central axis of the carrier.
The pressure on the front surface of the wafer may be modeled as being uniform with the pressure applied to the back surface of the wafer with the exception of the transitional areas between zones. The transitional areas are preferably described by an exponential function to more accurately reflect the actual pressure distribution on the front surface of the wafer. The exponential function of the transition area may be completely specified by the first derivative (slope) taken in the middle of the transition area. Each zone, except for the outermost zone, has two transitional areas allowing four slopes in total to be found for each zone.
Alternatively, a first derivative for the first material removal profile and a first derivative for the second material removal profiles may be calculated. Inputting the two points of intersection identifying an outer diameter of a zone, one at a time, into the two derivatives, one at a time, produces four slopes for each transition area.
Using either method, the absolute value of the four slopes may be averaged together to find the average absolute value of the first derivative for the points of intersection (RR1(X0)) for that zone. This number, which is different for every multizone carrier and wafer combination, allows a set of four equations to be solved that mathematically characterize the transition areas on the front surface of the wafer.